The present invention relates to plasma processing apparatus and methods, and more particularly to plasma processing apparatus and methods that use neutral or plasma sound waves to selectively control the location and concentration of plasma constituents, such as dust particles, reacting ions or molecules, and reaction products within the plasma.
In recent years, plasma processing has emerged as one of the most versatile and efficient techniques for the processing of materials in several of the largest manufacturing industries in the world. For example, in the electronics industry, plasma-based processes are indispensable for the manufacture of very large-scale integrated (VLSI) microelectronic circuits (or chips). Plasma processing is also a critical technology in the aerospace, automotive, steel, biomedical, and toxic waste management industries. For an overview of the many and varied applications that rely on plasma processing for materials processing, see, e.g., PLASMA PROCESSING OF MATERIALS Scientic Opportunities and Technological Challenges, National Research Council (National Academy Press, Washington, D.C. 1991).
In general, plasma processing involves the creation and maintaining of a plasma, and the application of the plasma to a particular material that is to be processed by the plasma. A plasma is a partially or fully ionized gas containing electrons, ions, and neutral atoms and/or molecules. In a typical plasma processing application, the nonlinear collective interactions of the electrically charged constituents with each other, with neutral atoms and molecules, and with electric and magnetic fields, are used to selectively process a particular material that is exposed to the plasma. For example, in a plasma etching application, the plasma is used to selectively etch a semiconductor wafer on which VLSI microelectronic circuits are being formed.
In plasma etching, and many other plasma processing applications, one of the technological challenges that must be addressed is the control of "particles" in the plasma. A "particle" is generally considered as a small piece of material that is larger than a cluster of a few molecules, but small enough to remain suspended in a fluid for a time. Most particles are not advantageous. When a particle is not advantageous to the plasma process it is referred to as a "contaminant". Dust particles are an example of contaminants that interfere with the delicate plasma etching operation used in making VLSI chips. See, e.g., Donovan, Particle Control For Semicondcutor Manufacturing (Marcel Dekker, Inc. New York 1990). The presence of a dust particle having a size less than about 10 .mu.m (where one .mu.m is 10.sup.-6 meters), for example, is problematical for most VLSI processing where circuit traces and other component sizes and spacings on the VLSI chip may only be on the order of 1-5 .mu.m or smaller. Hence, contaminants as small as 1 .mu.m may still play havoc with the precise etching that must be achieved in most VLSI processing applications. Thus, there is a critical need in the plasma processing art for a way to control the presence and/or location of contaminants in the plasma so that such contaminants do not interfere with the plasma processing operation that is being performed.
While filtering the gases that are used to create the plasma is effective at removing most large contaminant particles from a plasma, current filtering technology cannot remove all contaminant particles from the plasma formation gases. Thus, there will always be some medium size and smaller contaminant particles that are present in the plasma. Further, some contaminants are created as part of the plasma processing operation, i.e., they originate from the material being processed, so there is no way to effectively remove such contaminants from the plasma through filtering. Should such contaminant particles lodge on the surface of a VLSI chip, for example, they could easily short out the traces that form part of the VLSI circuit, thereby rendering such circuit inoperable. What is needed, therefore, is a way to insure that contaminant particles that are present in the plasma, from whatever source, are kept away from the surface of the material being processed, thereby preventing them from contaminating the surface of such material.
In addition to removing contaminants from the plasma processing operation, the plasma processing operation can be significantly enhanced (made more efficient) if the reactants in the plasma, e.g., ions and/or molecules, could be more heavily concentrated near the material being processed than elsewhere within the plasma. In this way the desired plasma processing reaction can occur faster. Unfortunately, such reactants are typically orders of magnitude smaller than the contaminant particles described above, and there is no known technique for controlling their concentration at a specific location in the plasma, i.e., near the material being processed, other than generally increasing their concentration throughout the entire plasma. In a typical plasma etching application, not only must the reactants be present in the plasma, but as a result of the reaction between the reactants and the material being processed, usually a chemical reaction, there are also "waste" products from the reaction that must be removed from the plasma. Usually such waste products are volatile, so they are pumped out of the plasma processing chamber. Disadvantageously, however, when the concentration of the reactants is increased, which concentration (absent the present invention) necessarily increases throughout the plasma, many of the reactants are also pumped out with the waste products, or otherwise removed from the plasma, thus limiting the efficiency of the plasma etching operation. What is needed, therefore, is a way to concentrate the reactants in the plasma near the surface or edge of the plasma where the reactants interact with the material being processed, away from the regions of the plasma where the reactants are not used and where such reactants might otherwise be removed from the plasma with the waste products.
Further, most plasma processing operations could benefit if the concentration of products resulting from the plasma processing reaction could be decreased, particularly in the region near the surface of the material being processed. That is, by decreasing the concentration of the products of reaction in the region where the plasma processing reaction is occurring, the desired plasma processing reaction can occur faster, and hence more efficiently. Thus, there is a need in the plasma processing art for a way to remove and/or decrease the concentration of the products of reaction in the region where the reaction takes place so that the desired plasma processing reaction can occur more efficiently.